Of the four major types of fat that we eat, polyunsaturated fat is the strangest. But it’s the type that those of us who have diabetes most need to take time to understand.

Last week I reviewed a huge -- and hugely important -- new study that vindicated saturated fat. That study, “Saturated Fat is Back for People with Diabetes,” analyzed data from 72 cohort studies and randomized trials with more than 600,000 participants from 18 countries and concluded that total saturated fat was not connected to the risk of heart disease.

That study also included polyunsaturated fats. But while the findings were straightforward about saturated fat, those concerning polyunsaturated fat were nuanced.

Saturated and polyunsaturated fats are the two types that we have misunderstood the most in this country. We now seem to know about the other two types, monounsaturated and trans fats. We have general agreement that monounsaturated fats, which we find in many foods including olive oil and avocados, promote good health. And during the previous decade we have generally grown to understand that those artificially created trans fats made from partially hydrogenated vegetable oils are about the worst “food” we can put in our mouths.

Polyunsaturated fats are more complex. But we have begun to understand that the two main sub-types, omega-3 and omega-6, compete to get into the cells of our bodies and have quite different effects.

Omega-6 fats are the ones that our doctors have been telling us to eat. Soybean oil is the dominant oil, followed by corn oil, canola oil, and cottonseed oil. These four make up 96 percent of all the vegetable oil sold in this country, according to Evelyn Tribole in her book The Ultimate Omega-3 Diet (McGraw Hill, 2007, pp. 28-29). In addition to cooking with them, they are in most margarine, shortening, and salad dressings.

The new study suggests that the two main types of long-chain omega-3 fats are associated with a lower risk of heart disease. These fats are eicosapentaenoic and docosahexaenoic acids, which almost nobody can pronounce, so we usually call them EPA and DHA. We get them almost exclusively from fatty fish like salmon and sardines.

This was no surprise. I’ve reported here on several earlier studies that extol the benefits to our hearts of eating fatty fish. What was a surprise was that one omega-6 fat, arachidonic acid, also seems to be associated with a reduced risk of heart disease. We get most of this fat from animal sources -- meat, eggs, dairy.

But the biggest surprise is that supplements of any of these polyunsaturated fats “do not statistically significantly reduce the risk for coronary outcomes.” In plain English it doesn’t seem that taking fish oil or krill oil will do us any of the good that we thought we were getting from them. Only by eating fatty fish, meat, eggs, and dairy will polyunsaturated fat help us to stave off heart disease. This is so important because heart disease is the single leading cause of death and disability for people with diabetes and in fact for all humans beings.

These findings about taking fish oil or krill oil supplements are tentative. The researchers write that “the available evidence in generally limited.” More studies are needed to make a firm conclusion, and fortunately two large randomized trials to see if taking long-chain omega-3 supplements can help prevent heart disease are underway. As soon as these studies get published, you can count on me to report on them here.